Anionic polymerization of oxiranes

Polyethylene oxide)macromonomers have thus been synthesized by two-step processes. Poly(oxyethylene)mo nomethyl ethers with widely varying molecular weights are commercially available. They are obtained by anionic polymerization of oxirane initiated by monofunctional alkoxides261 such as potassium 2-methoxyethanol. 

Attempts to synthesize the same type of compounds by anionic polymerization of oxiranes in the presence of transfer agents snch as HEMA failed because anionic polymerization requires temperatures at which thermally initiated polymerization of the methacrylic double bond occurs... 

The formation of aggregates with low to zero polymerization activity is quite general in ionic polymerizations. This statement can be further documented by the observation of centre aggregation during anionic polymerization of oxirane [100-103]... 

Ethylene Oxide The anionic polymerization of ethylene oxide is complicated by the association phenomenon and the participation of ion-pair and free ion intermediates in the propagation reactions [129, 130]. Simple lithium alkoxides are strongly associated into hexamers and tetramers even in polar media such as THE and pyridine [130]. As a consequence, lithium alkoxides are unreactive as initiators for the anionic polymerization of oxiranes. Association effects can be minimized by effecting polymerizations in alcohol media or in dipolar aprotic solvents. 

B. Esswein, N. M. Steidl and M. Moeller, Anionic polymerization of oxirane in the presence of the polyiminophosphazene base tert-Bu-P4, Macromol. Rapid Commun. 17 143 (1996). 

Oxiranes. Both anionic and cationic polymerization can be used to synthesize telechelics from oxiranes. Anionic polymerization of oxirane (ethylene oxide) yields a , )-dihydroxy poly(ethylene oxide) (274,275). 

Scheme 14 Phosphonium and phosphazene cations used as counterions for the anionic polymerization of oxiranes.

Anionic polymerization of oxirane requires the use of nucleophilic initiators and involves mostly alkali metal compounds, which are characterized by a high nudeophilidty of the monomer-attacking agent and by low Lewis acidity of the positive counterion. Thus, nudeophilic initiation of the anionic polymerization does not require any monomer coordination to the metal, although interaction of the monomer with an electrophilic counterion is considered commonly to take place to some extent, as it will be discussed below. 

Koinuma, H., Naito, K., Hirai, H., Anionic Polymerization of Oxiranes., Makromol. Chem., 1982,183,1383. 

The use of an unsaturated anionic initiator—such as potassium p-vinyl benzoxide—is possible for the ring opening polymerization of oxirane [43]. Although initiation is generally heterogenous, the polymers exhibit the molecular weight expected and a low polydispersity. In this case, the styrene type unsaturation at chain end cannot get involved in the process, as the propagating sites are oxanions. 

The anionic polymerization of 2-furyl oxirane 4a (17) proceeds smoothly and can be made to yield oligomers bearing a hydroxyl group at each end ... 

Although anionic polymerization of cyclic ethers is generally limited to oxiranes, there are reports of successful oxetane and tetrahydrofuran polymerizations in the presence of a Lewis acid. Aluminum porphyrin alone does not polymerize oxetane, but polymerization proceeds in the presence of a Lewis acid [Sugimoto and Inoue, 1999]. Similarly, THF is polymerized by sodium triphenylmethyl in the presence of a Lewis acid such as aluminum alkoxide [Kubisa and Penczek, 1999]. The Lewis acid complexes at the ether oxygen, which weakens (polarizes) the carbon-oxygen bond and enhances nucleophilic attack. 

The polymerization of oxiranes, a reaction of importance for both industry and commerce, has been abundantly described in the literature. Several hundred articles are published in this field annually. The quantity and great variety of themes discussed mean that a survey of this immense literature material exceeds the scope of the present review. Accordingly, we shall merely mention some of the works attempting to clarify the situation regarding the mechanism of polymerization reactions. ° We shall also outline the fundamental types of oxirane polymerizations. These can be classified into three groups with anionic, cationic, " and coordination mechanisms. 

It is widely recognized that Et Zn-H O system is one of the most active catalysts for the stereospecific polymerization of oxiranes. A variety of chemical species are formed in the following way rapid formation of ethylzinc hydroxide, its aggregation, and elimination of ethane to form zinc oxide structure. The maximum catalyst activity was achieved when the mole ratio of zinc to water was one to one, where the predominant formation of a species, Et(ZnO) H, (III), was observed. If we use less amount of water, another species, Et(ZnO) ZnEt, (IV), was also produced concurrently. Contrary to the anionic nature of the former species (III), the latter species (IV) exhibited a cationic nature. For instance, more than 957o of ring cleavage of methyloxirane takes place at O-CH bond with species (III), while the cleavage at 0-CH bond also takes place concurrently with species (IV). 

The ring-opening polymerization of oxiranes, thiiranes, and thietanes can be initiated by both cationic and anionic methods, but thrae are some heterocycUc compounds such as lactones and lactams that are more suited to the anionie technique. 

Aluminum porphyrins initiate controlled ring-opening polymerizations of oxiranes [67-69] ]3-lactones [70-72], 5-valerolactone [74], -caprolactone [74] and D-lactide [75], as well as controlled addition polymerizations of methacrylates [76] and methacrylonitrile [77] (Eq. 15). As shown in Eq. (16), propagation occurs by a coordinative anionic mechanism... 

Polymerizations of oxiranes or epoxides occur by one of three different mechanisms (1) cationic, (2) anionic, and (3) coordination. In this respect, the oxiranes differ from the rest of the cyclic ethers that can only be polymerized with the help of strong cationic initiators. It appears, though, that sometimes coordination catalysis might also be effective in polymerizations of some oxetanes. The susceptibility of oxirane compounds to anionic initiation can be explained by the fact that these are strained ring compounds. Because the rings consist of only three atoms, the electrons on the oxygen are crowded and are vulnerable to attack. ... 

Cationic polymerizations of oxiranes are much less isospecific and regiospecific than are anionic polymerizations. In anionic and coordinated anionic polymerizations, only chiral epoxides, like propylene oxide, yield stereoregular polymers. Both pure enantiomers yield isotactic polymers when the reaction proceeds in a regiospecific manner with the bond cleavage taking place at the primary carbon. 

The anionic polymerization of the oxirane ring which can be catalyzed by (I) and co-catalyzed by... 

Heterocyclic Monomers. A variety ofheterocycHc monomers can be polymerized by anionic ring-opening polymerizations. The types of anionically polymerizable heterocyclic monomers include oxiranes (epoxides), thiacyclopropanes, thiacyclobutanes, lactones, lactides, lactams, anhydrides, carbonates, and silox-anes (92). Among these heterocyclic monomers, the anionic polymerizations of epoxides have been examined most extensively. 

The anionic polymerization of methyl oxirane occurs predominantly by breaking of the oxygen-methylene bond, yielding terminal secondary alcohol functionality. 

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